So I am guessing you are using Windows Vista (any version) 64-bit and you just installed Quicktime for windows to discover a black bar instead of a progress bar with a mute button in Firefox when watching trailers or audio files… well people I had the exact same problem and here’s the fix:
Either go to your desktop icon for Firefox or go to the main Firefox folder which should be located in Computer/Local DIsk (C:)/Program Files (x86)/Mozilla Firefox
1. Right click on the browser icon
2. Select properties
3. Go to the compatibility tab
4. Check the box for “Run this program in compatibility mode for:”
5. Make sure it says “Windows XP (Service Pack 2)” in the drop down list.
6. Press “OK” at the bottom

Restart Firefox if it was launched while you did this.
Done!.

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